Resumen de The Variable Transition State in Polar Additions to Pi Bonds
A vast majority of polar additions of Brønsted acids to alkynes involve a termolecular transition state. With strong acids, considerable positive charge is developed on carbon and Markovnikov addition predominates. In less acidic solutions, however, the reaction is much slower and the transition state more closely resembles the olefinic product. For conjugated alkynes, the lower-energy transition state can retain the conjugated pi system by undergoing addition to the isolated orthogonal pi bond of the alkyne. As a result, anti-Markovnikov products can predominate.
